The present invention is concerned with a novel process for preparing acetals and ketals. As is known, acetals and ketals can be prepared by reacting an aldehyde or ketone with an alcohol in the presence of an acidic catalyst. However, the reaction is reversible and, at ambient temperature or above, the equilibrium of the reaction is shifted to the side of the starting materials, acetal or ketone, and alcohol.
The invention provides a novel process for the continuous preparation of acetals and ketals in concentrated form and avoids energy-intensive destination procedures of conventional manufacturing processes, which are often rendered difficult by the formation of azeotropes.
Thus, the invention is concerned with a process for the preparation of acetals or ketals which comprises reacting an aldehyde or ketone with an alcohol in the presence of solid acid and removing water from the reaction product by pervaporation.
More specifically, the present invention is concerned with a process for recovering acetals or ketals from reaction mixtures obtained by reacting an aldehyde or ketone with an alcohol, particularly by reaction of a lower aliphatic aldehyde or ketone with a lower aliphatic alcohol or sugar alcohol, in the presence of an acid which process comprises subjecting the reaction mixture containing an acetal or ketal together with water and unreacted aldehyde or ketone and alcohol, to treatment with a base followed by pervaporation.
In the following, the term xe2x80x9cketalxe2x80x9d and xe2x80x9cketalisationxe2x80x9d will be used to simultaneously denote acetals and acetalisation, respectively. The term xe2x80x9cketonexe2x80x9d includes ketones and aldehydes. The term xe2x80x9clowerxe2x80x9d as used herein denotes compounds having 1 to 7 carbon atoms. Examples of lower aliphatic ketones are acetone and methyl ethyl ketone. Examples of lower aliphatic aldehydes are formaldehyde, acetaldehyde, propionic aldehyde, butyric aldehyde and isobutyric aldehyde. Examples of alcohols are methanol and ethanol. Sorbose is an example of a sugar alcohol.
Pervaporation is a known method for separating liquids from mixtures thereof, e.g., for separating water from mixtures with organic liquids, such as alcohols, aldehydes or ketones, see, e.g., European Patent No. 0 096 339, and Chem. Eng. Technol. 19 (1996) 117-126. In pervaporation processes, the different ability of liquids or gases to permeate polymer membranes is used to separate mixtures thereof.
While pervaporation has been proposed to separate water e.g., from esterification reactions, the successful application of this method to remove reaction water from acetalisation or ketalisation processes has, so far, not been reported. This is not surprising since in ketalisation reactions the reaction product is in equilibrium with the starting ketone and alcohol, and low temperatures are required to shift the equlibrium to the side of the ketal. Pervaporation processes, to be carried out efficaciously, require elevated temperatures where the equilibrium of the ketalisation reaction is shifted markedly to the side of the starting materials of the reaction.
In a preferred embodiment the process of this invention is carried out in a number of consecutive steps. In a first step, an alcohol is reacted with an aldehyde or ketone in the presence of a solid acid to obtain an equilibrium mixture comprising the reactants, the desired ketal, and water. In a second step, the equilibrium mixture obtained is subjected to treatment with a solid base followed by pervaporation. In a third step the pervaporation retentate is subjected to treatment with a solid acid under conditions that favour ketalisation. In a fourth step, the product from the third step treated with a solid base followed by pervaporation. The removal of water from the pervaporation retentate is repeated until the ketal is obtained in the desired purity which is determined by the requirements of the ultimate use of the ketal, i.e. by the requirements of the reactions wherein the ketal is processed further.
The process of this invention can be applied to any ketalisation reaction. Examples of such reactions are
Conversion of acetone to 2,2-dimethoxy propane;
Conversion of methyl ethyl ketone to dimethoxy butane;
Conversion of sorbose to sorbose diacetonide;
Conversion of butendiol to isopropoxy dioxepen;
Conversion of methyl glyoxal to dimal.
In a more preferred aspect, the process of this invention is used to prepare 2,2-dimethoxy propane from acetone and methanol.
In the first step of the reaction in accordance with the invention the solid acid is suitably a strongly acidic polymer such as a polystyrene sulfonic acid, which may be macroporous or gel-type. Ion exchange resins conventionally used to catalyze ketalisation reactions can be used. Examples of such ion exchange resins are Dowex 50 (Dow Chemical), Amberlite IR 120, Amberlyst A 15 and A 36 (Rohm and Haas), Lewatit (Bayer). The reaction temperature is suitably from about xe2x88x9250xc2x0 to about 10xc2x0 C., preferably from about xe2x88x9235xc2x0 to about xe2x88x9240xc2x0 C.
Examples of bases as used in the second reaction step are weakly basic ion exchange resins such as polystyrenes resins carrying quaternary ammonium groups, e.g. IRA 96 (Rohm and Haas).
For the pervaporation, any membrane which is resistent to the reaction products and which are permeable for water may be used. Examples of such membranes are hydrophilic membranes which may be polymer or ceramic membranes. Polymer membranes may be composite membranes comprising a support layer, e.g. on the basis of acrylonitrile polymers, and a polyvinyl alcohol layer which provides the actual active separating layer. Examples of membranes useful in the process of this invention are membranes provided by Sulzer Chemtech GmbH, D-66540 Neunkirchen, Germany under the name Pervap 1055, Pervap 2000 and Pervap 2510; as well as membranes provided by CM-CELFA Membrantechnik AG, CH-6423 Seewen, Switzerland, under the name CMC-CE-02 and CM-CE-01.
The pervaporation is suitably carried out at elevated temperatures, i.e., temperatures up to the boiling point of the reaction mixture on the retentate side of the membrane. In general, the pervaporation is carried out at about 60xc2x0 to about 130xc2x0 C. The pressure in the pervaporation is not critical and is basically determined by the pressure required to sustain the mass flow. However elevated pressure, e.g., up to 4 Bar on the retentate side of the membrane can be used, subject to the mechanical resistance of the membrane, to increase the boiling point of the reaction mixture, thus allowing the pervaporation to proceed at higher temperature. The pressure on the permeate side of the membran is suitably about 1 to about 500 mBar.